fix(etl): normalize empty playlist_contents on update (apps#14306 parity)

[raymondjacobson]

Summary

Picks up the backend behavior change from apps#14306 ("allow removing the last track from a playlist").

apps' bug

In Python, if not playlist_metadata.get("playlist_contents") is true for an empty list. So when the SDK sent {"playlist_contents": []} (user removing the last track from a playlist), apps' populate_playlist_record_metadata silently skipped the JSONB update. The UI showed the track removed (optimistic update), but a reload restored it because the persisted state was unchanged.

apps' fix swaps if not ... for if ... is None, so the empty list goes through process_playlist_contents like any other value.

go-openaudio status

go-openaudio's mergePlaylistFromMetadata already uses a _, ok := key-exists check, not a truthiness check — so the underlying bug does not reproduce here. Verified via TestPlaylistUpdate_EmptyArrayMarksAllTracksRemoved: the playlist_tracks junction correctly marks all rows is_removed=true.

Adjacent gap found and fixed

apps' process_playlist_contents always normalizes its output to the dict form {"track_ids": [...]} regardless of input shape. go-openaudio was persisting whatever shape the SDK sent (bare [] or legacy dict), which downstream readers wouldn't expect.

This PR adds normalizePlaylistContentsJSON:

{}                                              → {"track_ids":[]}
{"playlist_contents": null}                     → {"track_ids":[]}
{"playlist_contents": []}                       → {"track_ids":[]}
{"playlist_contents": {"track_ids": []}}        → {"track_ids":[]}
{"playlist_contents": [{...}]}                  → {"track_ids":[{...}]}
{"playlist_contents": {"track_ids": [{...}]}}   → {"track_ids":[{...}]}

Wired into both create (metadataPlaylistContentsJSON) and update (mergePlaylistFromMetadata).

Stack context

Stacked on #252 (5E — oauth_redirect_uris). Follow-up after the main parity stack.

Test plan

• TestNormalizePlaylistContentsJSON — 6 sub-cases covering all input shapes.
• TestPlaylistUpdate_EmptyArrayMarksAllTracksRemoved — both bare [] (new SDK) and legacy {track_ids:[]} paths correctly clear the junction.
• TestPlaylistUpdate_EmptyArrayWritesJSONBColumn — asserts the persisted JSONB is a JSON object with a track_ids key (apps' canonical form).
• All previously-passing playlist tests still pass (no regressions).

🤖 Generated with Claude Code